The answer to this could be really simple or really complicated, depending on how deeply the issue is covered. Therefore, the author will dispense with the really complicated by limiting the discussion to solids, liquids, and gases. There are many other states of matter, including plasmas, superfluids, and Bose-Einstein condensates (!). Because this is a column and not a book, you’ll have to look up those other states of matter elsewhere. Even though we’re staying simple, you might get a surprise or two, such as the fact that it’s possible for iron to be a gas, and it’s possible to have liquid or even solid hydrogen.